Composite materials, particularly those obtained by blending carbon materials into polymer materials such as resin and rubber, have been conventionally used as materials with superior electrical conductivity and mechanical characteristics. Among carbon materials, carbon nanotubes (hereinafter “CNTs”) are now attracting attention because they highly improve electrical conductivity and mechanical characteristics. Specifically, a single-walled carbon nanotube (hereinafter “SWCNT”), a fibrous conductive filler that exhibits high electrical conductivity, is attracting particular attention as a carbon material of composite materials. SWCNT can improve the electrical conductivity and mechanical characteristics of the composite materials well even when the blending amount thereof is small.
Carbon materials or CNTs need to be uniformly dispersed in a matrix of a polymer material to achieve improved electrical conductivity and mechanical characteristics in the composite material well. The following technique has been proposed in the art: a CNT dispersion liquid obtained by uniformly dispersing CNTs in a solvent and a polymer material are mixed together to form a composite material composition, which is then prepared into a composite material where CNTs are uniformly dispersed in a matrix of the polymer material.
However, CNTs such as SWCNTs and the like having a large specific surface area tend to aggregate and tangle one another. Thus, the production of composite materials that include CNTs as a carbon material requires a technique of efficiently preparing a CNT dispersion liquid in which CNTs are uniformly dispersed.
To achieve this, JP2006016222A (PTL 1) for example proposes breaking coarsely-dispersed CNTs in a solvent using shear forces, shock waves, cavitation, and the like. This highly disperses the broken CNTs in a CNT dispersion liquid. JP2010254546A (PTL 2) proposes cutting and untangling aggregated CNTs in a solvent using an ultrasonic homogenizer. This also highly disperses CNTs in a CNT dispersion liquid.